Titan chief executive Maurice Taylor's incendiary remarks came as France's economy is struggling in the face of increasing global competition, with leading companies announcing thousands of job cuts in recent months.
The country's Socialist government has vowed to tackle France's productivity gap -- blamed by critics on high wages and reduced working hours -- but is facing opposition from powerful labour unions.
The letter from Taylor to French Industrial Renewal Minister Arnaud Montebourg was in response to a request for Titan to consider investing in a loss-making Goodyear tyre plant in Amiens, northern France.
"I have visited that factory a couple of times. The French workforce gets paid high wages but only works three hours," Taylor said in the letter, dated February 8 and obtained by French business daily Les Echos.
"They get one hour for breaks and lunch, talk for three and work for three. I told this to the French union workers to their faces. They told me that's the French way!"
Goodyear said last month it was set to close the plant, which employs 1,173 workers, following five years of failed talks with unions.
Taylor said Titan had a long history of buying and turning around troubled factories but in this case was not in any way interested.
"Sir, your letter states that you want Titan to start a discussion. How stupid do you think we are? Titan is the one with the money and the talent to produce tyres. What does the crazy union have? It has the French government," Taylor wrote.
The standard length of the working week in France is 35 hours.
The letter drew a furious reaction from unions.
"This is an insulting letter," said Mickael Wamen, the CGT union's representative at the Goodyear plant, saying it showed Taylor "belongs more in an insane asylum than at the head of a multinational corporation".
He said the union was planning to file a lawsuit in the United States against Goodyear and Titan over the closure of the plant.
Titan had one point been touted as a potential saviour of the factory, which produces tyres for agricultural vehicles and which Goodyear says it was forced to close after unions repeatedly rejected efforts to cut costs.
France's Communist Party also expressed outrage at the letter, calling it "an appalling provocation coupled with xenophobia against French workers."
It urged the government to pass a law forbidding profitable companies from laying off workers.